1. Field of the Invention
This invention relates generally to forming of molten metal; and more particularly to very efficient apparatus and method for forming either small investment castings or ingots. The invention is especially advantageous in casting of extremely reactive and refractory metals such as titanium, but is not limited to such use.
2. Prior Art
Titanium and like metals present a great challenge to efficient manipulation and forming, since they can be heated only with special precautions to prevent both contamination of the metal and damage to the environment. This is particularly true in making so-called "investment" castings, and even in forming ingots, because these metals can be safely melted only in inert or highly rarified atmospheres.
Prior patents in the field of forming molten metal, and particularly forming molten titanium and the like, may be very roughly divided into tilt-to-pour, bottom-pour, and countergravity systems. Each has its own drawbacks.
Tilt-to-pour systems are generally very cumbersome and costly, since moving machinery for the tilting of crucibles and other handling of molten metal must be provided inside an evacuable sealed chamber. Furthermore, the complete filling of a mold can be very difficult, especially when the mold has small intricate details to be formed in the metal.
Such difficulty arises because gas bubbles trapped in the finely detailed parts of the mold deter entry of the molten metal, and the metal when incompletely molten is viscous and sometimes inhomogeneous. If the chamber is operated at very low pressures to avoid gas bubbles, yet other complexities arise.
Typical of such highly elaborate systems is that described in U.S. Pat. No. 3,658,119 to Hunt. In that apparatus electron beams heat and melt a charge in a crucible, and the crucible is tilted to pour the molten charge. Other electron beams keep the metal molten while it proceeds along chutes or conveyors.
Hunt's system is used for forming either useful articles--by which I mean end-product articles, such as dental castings, for use outside the casting industry itself--or ingots for remelting in other steps to make useful articles. Hunt takes a direct approach to loading his furance: a conveyor drops particulate metal into the crucible.
U.S. Pat. No. 4,154,286 of Glazunov illustrates one technique for achieving a complete fill of the mold. His crucible tilts to pour into a "pressmould," a device with a piston that drives molten metal into the mold.
Another U.S. Pat. No. 3,807,488 to the same inventor shows how to make thin-walled workpieces centrifugally, by spinning a large round mold that receives molten metal from a tilted crucible. In the device of each Glazunov patent, a consumable electrode drips molten metal into the crucible.
Yet another tilt-to-pour system is described in U.S. Pat. No. 3,273,212 of Garmy. That system too uses a consumable electrode, but further allows additional particulate charge to be fed from above as in Hunt. As to mode of loading it is thus a hybrid system. Garmy also teaches use of the "skull"--the unmelted solid portion of the metal charge in the crucible--as a noncontaminating liner for the crucible.
One step more sophisticated than the tilt-to-pour technique is the bottom-extraction or bottom-pour strategy, which is mechanically advantageous in that the crucible need not be bodily tilted. The general principle here is to provide an opening at the bottom of the crucible and simply move metal downward out of that opening.
In making ingots, it is not generally necessary to shape the top of an ingot narrower than the lower portions. This fact provides a degree of freedom not generally available in making castings.
Hence U.S. Pat. No. 3,894,573 of Paton makes shaped ingot by dripping metal from a consumable electrode onto a piston-supported floor in a vertical cylindrical form. While continuing to operate the melting arc, Paton continuously "extracts" formed ingot at the bottom by retracting the piston.
As in two of the patents already mentioned, Paton suggests that scrap metal may be remelted into the pool of molten metal by loading from above. When the elevator is fully lowered, he opens the bottom of the form, removes the tall cylindrical ingot, and reseals the apparatus for the next cycle.
U.S. Pat. No. 4,197,900 of Bloshenko discloses a similar ingot form with a detachable bottom plate. One may infer, although the disclosure does not so state, that ingot is removed at the bottom of the form.
Similar ingot-forming systems are known in which only top-loaded particulate makes up the charge, which is melted by a nonconsumable electrode. Thus early U.S. Pat. No. 2,727,937 of Boyer discloses a movable mold floor on an elevator. The floor is lowered during operation, as in Paton, to maintain the arc length.
The charge is added by dropping particles from a hopper. After operation the elevator is lowered further for bottom removal of finished ingot. Boyer's nonconsumable electrode is a water-cooled copper coil.
Such prior-art systems are generally reliable and efficient for forming good-quality ingot. The prior art has failed, however, to produce adequate systems for making castings; and has accordingly failed to produce adequate systems that can be used for both castings and ingot.
Unfortunately, as suggested above, the problem of finished-part removal is more complicated and difficult for castings than for ingot. This fact is illustrated by consideration of the bottom-pour casting systems disclosed by Waterstrat in U.S. Pat. Nos. 4,538,671 and 4,627,482.
In these patents it is indicated that premature release of molten metal into a mold must be prevented by some sort of valving function. Waterstrat shows both a fusible foil and a mechanical valve as alternatives for use in the valving function.
As to the mechanical valve, a relatively coarse valve action is desirable since molten titanium and the like may be expected to clog and possibly obstruct the operation of most sorts of precision-acting valve. Clogging is likely to be especially troublesome if water cooling of the valve is provided. If the valve is not cooled, however, then contamination of the melt and short equipment life may be expected instead.
Waterstrat apparently prefers the fusible foil, since it permits the mouth of the mold to be sealed around the melt-flow channel from the crucible. This in turn facilitates evacuating the volume around the mold, to obtain a pressure-differential assist to complete filling of the mold.
In both cases--whether foil or valve--there results a constriction in the system, between the pool of molten metal and the interior of the mold. In many situations the mold itself defines such a constriction anyway, but in the Waterstrat system even the best efforts of the mold designer cannot avoid a constriction.
Thus there is an inherent difficulty with the Waterstrat configuration: if the molten metal is allowed to cool both above and below the constriction, while the pouring channel is open, the molded part will thereby be frozen to the casting apparatus.
On the other hand, if this trap is to be avoided, either the valve must close when the mold is just exactly full, or the charge must be exactly the right size so that--when melted--it just fills the mold (subject to tolerances provided by the length of the flash channel).
The first of these solutions militates against using a fusible foil, since such a foil once fused cannot be closed again. The foil is relatively undesirable anyway, since it actually melts into the molten charge and so constitutes a source of contamination to the part being cast; furthermore a new foil must be installed with each new charge, introducing additional cost and delay.
If the mechanical valve is used, however, then sealing around the mouth of the mold becomes very awkward. The previously mentioned coarse action of the valve--which is evident in Waterstrat's illustrations--appears generally incompatible with establishment of a clean, clog-free and contaminant-free flow channel for the melt.
The second solution suggested above (making the charge exactly the right size) has its own problems. For one, the top of the system must be opened and a complete new charge provided for each new mold. For another, small changes in operating conditions may throw off the relationship between charge size and mold volume--as, for example, by changing the volume of the flash channel. For still another problem, some production lines make different-size parts at each cycle, and each solid charge must therefore be matched to its mold.
The operation of opening the top of the system to install a new charge is particularly objectionable. That operation must be performed in addition to opening the bottom of the system to position the mold.
Every added operation that must be performed to prepare the furnace for making the next casting is a serious matter. The economics of operating refractory-metal casting furnaces demand high throughput of parts.
Additional process steps elsewhere in the production line may be readily tolerable. The furnace, however, is usually the point of greatest capital investment and therefore in terms of economics is the process bottleneck.
Nevertheless at least one other patent echoes Waterstrat's precautions against premature release of the molten metal. U.S. Pat. No. 4,471,831 to Ray describes a bottom-pour system for making metal ribbon. Ray says it is one of his objectives to make "the charge fully melted before the molten material is ejected."
These concerns over premature release are readily understood, and in fact are important. In a system of the Waterstrat type, if the charge is not fully melted, globules of viscous metal are likely to plug the mold--either in its necked-down entryway (the flash channel), or in finely detailed internal features that are provided to form details of the casting.
Ray's system is free of these particular problems, since he pours molten metal through the bottom of his crucible onto the periphery of a rotating vertical wheel. Still he discloses three different liquid-metal release systems for obtaining these objectives: (1) a valve with a shutter, (2) a low-melt plug, and (3) a valve with a stopper rod.
All three of these release systems pose significant contaminante problems. Furthermore the low-melt plug, like Waterstrat's fusible foil, is not reusable; and the two valves are subject to clogging or obstruction of the mechanisms, or both.
As already suggested, the ultimate potential problem with all such valving systems is in the possibility of the flash solidifying while it is still continuous with the melt in the crucible. Such solidification can result in lockup of the entire system--possibly requiring disassembly of the valve to obtain release.
It is therefore natural that Ray, in particular, provides an auxiliary heater in the nozzle. The prior art, in summary, teaches that (1) a controllable valving function is required to prevent premature release of metal that might be incompletely melted; and (2) the flash must be prevented from solidifying in the valve.
The foregoing discussion makes clear that the metal-release technique is critical to success of a casting system, or a system capable of making both casting and ingot. The prior art lacks a release system that is contaminant free, is reliable and smooth in operation, is recyclable quickly and economically, is amenable to provision of a pressure differential to aid in rapid, complete filling of the mold, and of course prevents premature release of only partially melted metal.
One type of known metal-casting system does satisfy nearly all these requirements; unfortunately, however, it appears practical only for very specialized applications. In that type of system, gravity acts as the "valve" closure, and suction acting against gravity acts as the release.
Typical of such "countergravity casting" systems is U.S. Pat. No. 4,658,880 of Voss. Voss's mold is made porous, and is inverted from a conventional orientation--so that the flash channel of the mold is at the bottom.
In operation, the open end of the flash channel is first immersed into a molten pool of the metal to be cast. Then a partial vacuum is applied to the exterior of the porous mold, to draw liquid metal up into the mold and against its porous wall.
Countergravity systems have the interesting property that unused material can be allowed to simply run back out of the mold and into the supply pool. They also have the properties that the charge in the supply pool is loaded from above, and "pours" upward from the pool into the mold; and that, after cooling, the mold and casting are removed upward from the pool.
In other words, the system is top-loading, top-pouring and top-unloading. This unusual combination of characteristics facilitates a relatively unencumbered and therefore efficient movement of material, molds and finished parts.
Such systems are particularly well adapted for making thin-walled, rough-interior castings. These systems do not appear feasible for general application such as investment casting of dental work or other intricate articles.
To completely fill all the detailed features in a mold by vacuum countergravity pouring is probably difficult. As will be appreciated, once the pores of the mold surface are full and the metal has begun to cool, the upward draw of the suction is greatly reduced. With this exception, however, countergravity systems do have essentially all the desirable characteristics enumerated above.
Another type of metal-melting system that is in actual use, though I know of no patent covering it, is an induction-heated pour-cup system. As far as I know, it is used only for nickel-based alloys, never for reactive or refractory metals, and the cup is simply a ceramic material rather than graphite or water-cooled metal as in titanium work and the like.
That system is remote from those already discussed, and is of interest only because it is in a sense bottom loading: the cup is advanced into the induction coil from below. The cup can be either tilted (together with the induction coil) to pour, or provided with a bottom-pour aperture.